Radiotherapeutic apparatus is often provided with a secondary diagnostic x-ray source in addition to the primary therapeutic source of x-rays (or other radiation). As the primary source is required to rotate around the patient in order to irradiate the target volume from a range of directions and thereby minimise the dose to healthy tissue, it is convenient to mount the diagnostic source on the same rotateable support as the primary source so that it too can rotate around the patient. In this way, the diagnostic source can produce a range of images of the patient from different directions, which can (if desired) be used to reconstruct a CT dataset.
Generally, there will be a flat panel imager associated with the secondary diagnostic source in order to capture the x-ray signal after attenuation by the patient. It is also common to provide a flat-panel detector for the therapeutic beam, to provide an image of the beam after attenuation by the patient. The two flat panel imagers will usually be somewhat different, each being suited to the different energies of the respective beams. Typically, the diagnostic beam is of a relatively lower energy (several kV), at which the attenuation coefficients of different tissue types are distinctly different and a good image with adequate contrast can be obtained. The therapeutic beam is of significantly higher energy (several MV), chosen for its therapeutic efficacy; at this energy the attenuation coefficients are more similar and such “portal” images are traditionally of poor contrast, although recent improvements in flat panel imagers has addressed this.
The result of this is that the rotateable support must carry two sources and two flat panel imagers, each opposite their associated source. Generally, this is achieved by placing the two sources at locations offset 90° from each other. Each source issues a pulse of radiation, and the associated imager receives a synchronised signal instructing it to read out an image frame. The flat panel imagers are usually of an integrating nature, i.e. each pixel output reflects the total amount of radiation that has impinged on that pixel location since the last read-out.